I have an isolation transformer with it's own power switch attached so I can turn the monitor power on and off without turning off the game pcb. I have different power adapters depending on the monitor I am trying to test.
I have a switching power supply with a JAMMA harness attached, also on it's own power switch. I wired in a switch to flip between positive and negative sync, depending on what board I am testing. I build JAMMA adapters for testing non-JAMMA boards, and have a rebuilt 13" monitor that I use on the bench when testing game pcb's. I also have an old X-Arcade control panel that I stripped out and rewired to use as a test control panel.
I have a partially functioning Blitz '99 PCB that has an on-screen test menu with different monitor test screens: color bars, cross hatch, and solid color screens among them. It has a switch to change the resolution between standard and medium, depending on what monitor I am testing at the time. I keep several tubes of the common chassis types handy so I can test and troubleshoot everything on my bench until it's fixed. I have a full-length mirror on the wall to one side of my bench so I can look over the monitor at the picture whenever I wish.
For something like X/Y monitors, it's a bit more difficult to build test bench setups, due to the different voltages and such, so I usually connect those to the game. Extension cables are great for having a monitor on the bench, but still connecting it to the cabinet. If you don't have one, I'll usually turn the cabinet so the back is easily accessible, then set up a small table that I can set the monitor on while testing it.
Bear in mind that pulling entire monitors for rebuilds are usually only necessary for something that is resisting repair using the easy methods (new caps, flyback, HOT, VR, and fixing cold solder joints). Otherwise, if the monitor has signs of life and you just need to freshen it up, then you can just pull the chassis, rebuild it on your bench, then put it back in and use a mirror to make adjustments.
And it's always a good idea to meet other local collectors, especially those with large collections. Then you can always take stuff over to their games and test them, just in case something else in your cabinet is causing your problem....